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Journal Articles

Time-reversal symmetry breaking phase and gapped surface states in -wave nanoscale superconductors

Nagai, Yuki; Ota, Yukihiro*; Tanaka, Kaori*

Journal of Physics; Conference Series, 969, p.012039_1 - 012039_6, 2018/03

https://doi.org/10.1088/1742-6596/969/1/012039

Times Cited Count：1 Percentile：52.81(Physics, Applied)

no abstracts in English

Journal Articles

Time-reversal symmetry breaking and gapped surface states due to spontaneous emergence of new order in -wave nanoislands

Nagai, Yuki; Ota, Yukihiro*; Tanaka, Kaori*

Physical Review B, 96(6), p.060503_1 - 060503_5, 2017/08

https://doi.org/10.1103/PhysRevB.96.060503

Times Cited Count：9 Percentile：42.75(Materials Science, Multidisciplinary)

no abstracts in English

Journal Articles

Nuclear magnetic relaxation rates of unconventional superconductivity in doped topological insulators

Nagai, Yuki; Ota, Yukihiro*

Physical Review B, 94(13), p.134516_1 - 134516_8, 2016/10

https://doi.org/10.1103/PhysRevB.94.134516

Times Cited Count：6 Percentile：30.11(Materials Science, Multidisciplinary)

no abstracts in English

Journal Articles

Critical temperature enhancement of topological superconductors; A Dynamical mean-field study

Nagai, Yuki; Hoshino, Shintaro*; Ota, Yukihiro*

Physical Review B, 93(22), p.220505_1 - 220505_5, 2016/06

https://doi.org/10.1103/PhysRevB.93.220505

Times Cited Count：8 Percentile：37.83(Materials Science, Multidisciplinary)

no abstracts in English

Journal Articles

A Screened automated structural search with semiempirical methods

Ota, Yukihiro; Ruiz-Barragan, S.*; Machida, Masahiko; Shiga, Motoyuki

Chemical Physics Letters, 648, p.119 - 123, 2016/03

https://doi.org/10.1016/j.cplett.2016.02.013

Times Cited Count：5 Percentile：19.3(Chemistry, Physical)

We developed an interface program between a program suite for an automated search of chemical reaction pathways, GRRM, and a program package of semiempirical methods, MOPAC. A two-step structural search is proposed as an application of this interface program. A screening test is first performed by semiempirical calculations. Subsequently, a reoptimization procedure is done by ab initio or density functional calculations. We apply this approach to ion adsorption on cellulose. The computational efficiency is also shown for a GRRM search. The interface program is suitable for the structural search of large molecular systems for which semiempirical methods are applicable.

Journal Articles

Theory of low-energy behaviors in topological s-wave pairing superconductors

Ota, Yukihiro; Nagai, Yuki; Machida, Masahiko

Physica C, 518, p.5 - 9, 2015/11

https://doi.org/10.1016/j.physc.2015.08.002

Times Cited Count：0 Percentile：0(Physics, Applied)

Topological superconductors are notable materials, owing to the mathematical interest as topological matter and the application potential of quantum engineering. To clarify their characters relying on gap-function types, we build up a low-energy effective theory, focusing on a model of superconductor Cu $_{x}$ Bi $_{x}$ Se $_{3}$ . The non-magnetic impurity effects in our previous numerical study indicates that the system has both p-wave and s-wave aspects. A perturbation study is insightful for explaining this peculiar behavior. Since the normal electrons are dominated by the Dirac-type dispersion relation, we assign a low-energy case as a large-Dirac-mass one. When the system has an odd-parity fully-gapped pair potential, our approach indicates that the effective gap function in the low-energy domain has not only a chiral p-wave-like component, but also an s-wave-like one. This peculiar mixture of p-wave and s-wave leads to intriguing responses in the present model.

Journal Articles

Inverse coherence effects in nuclear magnetic relaxation rates as a sign of topological superconductivity

Nagai, Yuki; Ota, Yukihiro; Machida, Masahiko

Physical Review B, 92(18), p.180502_1 - 180502_5, 2015/11

https://doi.org/10.1103/PhysRevB.92.180502

Times Cited Count：3 Percentile：14.69(Materials Science, Multidisciplinary)

no abstracts in English

Journal Articles

Topological -wave pairing superconductivity with spatial inhomogeneity; Midgap-state appearance and robustness of superconductivity

Nagai, Yuki; Ota, Yukihiro; Machida, Masahiko

Journal of the Physical Society of Japan, 84(3), p.034711_1 - 034711_8, 2015/03

https://doi.org/10.7566/JPSJ.84.034711

Times Cited Count：6 Percentile：45.25(Physics, Multidisciplinary)

no abstracts in English

Journal Articles

Robustness against non-magnetic impurities in topological superconductors

Nagai, Yuki; Ota, Yukihiro; Machida, Masahiko

Journal of Physics; Conference Series, 568(2), p.022030_1 - 022030_5, 2014/12

https://doi.org/10.1088/1742-6596/568/2/022030

Times Cited Count：0 Percentile：0.05(Physics, Applied)

We study the robustness against non-magnetic impurities in a three-dimensional topological superconductor, focusing on an effective model (massive Dirac Bogoliubov-de Gennes (BdG) Hamiltonian with -wave on-site pairing) of Cu $_{x}$ Bi $_{2}$ Se $_{3}$ with the parameter set determined by the first-principles calculation. With the use of the self-consistent -matrix approximation for impurity scattering, we discuss the impurity-concentration dependence of the zero-energy density of states. We show that a single material variable, measuring relativistic effects in the Dirac-BdG Hamiltonian, well characterizes the numerical results. In the nonrelativistic limit, the odd-parity fully-gapped topological superconductivity is fragile against non-magnetic impurities, since this superconductivity can be mapped onto the -wave superconductivity. On the other hand, in the ultrarelativistic limit, the superconductivity is robust against the non-magnetic impurities, since the effective model has the -wave superconductivity. We derive the effective Hamiltonian in the both limit.

Journal Articles

Impurity effects in a two-dimensional topological superconductor; A Link of $T_{rm c}$ -robustness with a topological number

Nagai, Yuki; Ota, Yukihiro; Machida, Masahiko

Journal of the Physical Society of Japan, 83(9), p.094722_1 - 094722_5, 2014/09

https://doi.org/10.7566/JPSJ.83.094722

Times Cited Count：9 Percentile：55.36(Physics, Multidisciplinary)

no abstracts in English

Journal Articles

Ferromagnetism in multi-orbital Fermi gas loaded on a one-dimensional optical lattice

Kobayashi, Keita; Okumura, Masahiko; Ota, Yukihiro*; Yamada, Susumu; Machida, Masahiko

JPS Conference Proceedings (Internet), 3, p.016006_1 - 016006_6, 2014/06

https://doi.org/10.7566/JPSCP.3.016006

no abstracts in English

Journal Articles

Nonmagnetic impurity effects in a three-dimensional topological superconductor; From - to -wave behaviors

Nagai, Yuki; Ota, Yukihiro; Machida, Masahiko

Physical Review B, 89(21), p.214506_1 - 214506_6, 2014/06

https://doi.org/10.1103/PhysRevB.89.214506

Times Cited Count：23 Percentile：67.92(Materials Science, Multidisciplinary)

Unconventional features in superconductivity are revealed by responses to impurity scattering. We study nonmagnetic impurity effects in a three-dimensional topological superconductor, focusing on an effective model (massive Dirac Hamiltonian with s-wave on-site pairing) of copper-doped bismuth-selenium compounds. Using a self-consistent T-matrix approach for impurity scattering, we examine in-gap states in density of states. We find that the results are well characterized by a single material variable, which measures relativistic effects in the Dirac Hamiltonian. In the nonrelativistic regime, an odd-parity superconducting state is fragile against nonmagnetic impurities. We show that this behavior is caused by a p-wave character involved in the topological superconducting state. In contrast, we show that in the relativistic regime the superconductivity is robust against nonmagnetic impurities, owing to an s-wave character. To summarize, the system has two aspects, p- and s-wave features, depending on the weight of relativistic effects.

Journal Articles

Theory of macroscopic quantum tunneling with Josephson-Leggett collective excitations in multiband superconducting Josephson junctions

Asai, Hidehiro*; Ota, Yukihiro; Kawabata, Shiro*; Machida, Masahiko; Nori, F.*

Physical Review B, 89(22), p.224507_1 - 224507_7, 2014/06

https://doi.org/10.1103/PhysRevB.89.224507

Times Cited Count：9 Percentile：38.87(Materials Science, Multidisciplinary)

Collective excitations reveal fundamental properties and potential applications of superconducting states. We study macroscopic quantum tunneling (MQT) in a Josephson junction composed of multiband superconductors, focusing on a phase mode induced by interband fluctuations: the Josephson-Leggett (JL) collective excitation mode. We derive a formula for the MQT escape rate for low-temperature switching events, carefully treating this collective mode. We clarify that the JL mode has two major effects on the MQT: (1) the zero-point fluctuations enhance the escape rate, and (2) the quantum dissipation induced by the couplings to the gauge-invariant phase difference suppresses the quantum tunneling. We show that the enhancement exceeds the suppression for a wide range of junction parameters. This enhancement originates from the single-mode interaction between the tunneling variable and the interband fluctuations.

Journal Articles